Not applicable.
This invention relates generally to equipment utilized in the transmission and distribution of electrical power. More specifically, the invention relates to transformers and other apparatus containing dielectric fluids, particularly dielectric fluids comprising relatively pure blends of compounds selected from the group consisting of aromatic hydrocarbons, polyalphaolefins, polyol esters, and natural vegetable oils. The invention further relates to the methods for preparing and processing such fluids and filling and sealing electrical apparatus with such fluids.
Many types of conventional electrical equipment contain a dielectric fluid for dissipating the heat that is generated by energized components, and for insulating those components from the equipment enclosure and from other internal parts and devices. Examples of such equipment include transformers, capacitors, switches, regulators, circuit breakers and reclosers. A transformer is a device that transfers electric power from one circuit to another by electrical magnetic means. Transformers are used extensively in the transmission of electrical power, both at the generating end and the user""s end of the power distribution system. A distribution transformer is one that receives electrical power at a first voltage and delivers it at a second, lower voltage.
A distribution transformer consists generally of a core and conductors that are wound about the core so as to form at least two windings. The windings (also referred to as coils) are insulated from each other, and are wound on a common core of magnetically suitable material, such as iron or steel. The primary winding or coil receives energy from an alternating current (AC) source. The secondary winding receives energy by mutual inductance from the primary winding and delivers that energy to a load that is connected to the secondary winding. The core provides a circuit or path for the magnetic lines of force (magnetic flux) which are created by the alternating current flow in the primary winding and which induce the current flow in the secondary winding. The core and windings are typically retained in an enclosure for safety and to protect the core and coil assembly from damage caused by the elements or vandalism.
The transformer windings or coils themselves are typically made of copper or aluminum. The cross section of the conductors forming the coil must be large enough to conduct the intended current without overheating. For small transformers, those rated less than 1 kVA, the coil wire may be insulated with shellac, varnish, enamel, or paper. For larger units, such as transformers rated 5 kVA and more, the conductor forming the coil is typically insulated with oil-impregnated paper. The insulation must provide not only for normal operating voltages and temporary overvoltages, but also must provide the required insulative levels during transient overvoltages as may result from lightning strikes or switching operations.
Distribution transformers used by the electric utilities in the United States operate at a frequency of 60 Hz (cycles per second). In Europe, the operating frequency is typically 50 Hz. Where the size and weight of the transformer are critical, such as in aircraft, transformers are typically designed to operate at a frequency of from 400 to 4,000 cycles per second. These high frequency applications allow the transformer to be made smaller and lighter than the 50 Hz and 60 Hz transformers designed for power distribution by the electric utilities.
The capacity of a transformer to transmit power from one circuit to another is expressed as a rating and is limited by the permissible temperature rise during operation. The rating of a transformer is generally expressed as a product of the voltage and current of one of the windings and is expressed in volt-amperes, or for practical purposes, kVA (kilovolt-amperes). Thus, the kVA rating of a transformer indicates the maximum power for which the transformer is designed to operate with a permissible temperature rise and under normal operating conditions.
Modern transformers are highly efficient, and typically operate with efficiencies in the range of 97-99%. The losses in the transformation process arise from several sources, but all losses manifest themselves as heat. As an example of the heat that is generated by even relatively small, fluid-filled distribution transformers, it is not uncommon for a 15 kVA mineral oil-filled transformer to operate with temperatures inside the transformer enclosure exceeding approximately 90xc2x0 C. continuously.
A first category of losses in a transformer are losses resulting from the electrical resistance in the conductors that constitute the primary and secondary windings. These losses can be quantified by multiplying the electrical resistance in each winding by the square of the current conducted through the winding (typically referred to as I2R losses).
Similarly, the alternating magnetic flux (or lines of force) generates current flow in the core material as the flux cuts through the core. These currents are referred to xe2x80x9ceddy currentsxe2x80x9d and also create heat and thus contribute to the losses in a transformer. Eddy currents are minimized in a transformer by constructing the core of thin laminations and by insulating adjacent laminations with insulative coatings. The laminations and coatings tend to present a high resistance path to eddy currents so as to reduce the current magnitudes, thereby reducing the I2R losses.
Heat is also generated in a transformer through an action known as xe2x80x9chysteresisxe2x80x9d which is the friction between the magnetic molecular particles in the core material as they reverse their orientation within the core steel which occurs when the AC magnetic field reverses its direction. Hysteresis losses are minimized by using a special grade of heat-treated, grain-orientated silicon steel for the core laminations to afford its molecules the greatest ease in reversing their position as the AC magnetic field reverses direction.
Although conventional transformers operate efficiently at relatively high temperatures, excessive heat is detrimental to transformer life. This is because transformers, like other electrical equipment, contain electrical insulation which is utilized to prevent energized components or conductors from contacting or arcing over to other components, conductors, structural members or other internal circuitry. Heat degrades insulation, causing it to loose its ability to perform its intended insulative function. Further, the higher the temperatures experienced by the insulation, the shorter the life of the insulation. When insulation fails, an internal fault or short circuit may occur. Such occurrences could cause the equipment to fail. Such failures, in turn, typically lead to system outages. On occasion, equipment can fail catastrophically and endanger personnel who may be in the vicinity. Accordingly, it is of utmost importance to maintain temperatures within the transformer to acceptably low levels.
To prevent excessive temperature rise and premature transformer failure, distribution transformers are generally provided with a liquid coolant to dissipate the relatively large quantities of heat generated during normal transformer operation. The coolant also functions to electrically insulate the transformer components and is often therefore referred to as a dielectric coolant. A dielectric coolant must be able to effectively and reliably perform its cooling and insulating functions for the service life of the transformer which, for example, may be up to 20 years or more. The ability of the fluid and the transformer to dissipate heat must be such as to maintain an average temperature rise below a predetermined maximum at the transformer""s rated kVA. The cooling system must also prevent hot spots or excessive temperature rises in any portions of the transformer. Generally, this is accomplished by submerging the core and coil assembly in the dielectric fluid and allowing free circulation of the fluid. The dielectric fluid covers and surrounds the core and coil assembly completely and fills all small voids in the insulation and elsewhere within the enclosure where air or contaminants could otherwise collect and eventually cause failure of the transformer.
As the core and coil assembly is heated, the heat is transferred to the surrounding dielectric fluid. The heated fluid transfers the heat to the tank walls and ultimately to the surrounding air. Most conventional distribution transformers include a headspace of air or inert gas, such as nitrogen, above the fluid in the tank. The headspace allows for some expansion of the dielectric fluid which will occur with an increase in temperature. Unfortunately, the headspace is also a thermal insulator and prevents or diminishes effective heat transfer from the fluid to the tank""s cover, since the cover is not xe2x80x9cwetted,xe2x80x9d meaning it is not in contact with the fluid. In such designs, because the cover or the top of the transformer tank provides relatively little heat transfer or cooling, the cooling must be sustained by the other surfaces of the enclosure that are in contact with the fluid.
In order to improve the rate of heat transfer from the core and coil assembly, transformers may include a means for providing increased cooling, such as fins on the tank that are provided to increase the surface area available to provide cooling, or radiators or tubes attached to the tank that are provided so that the hot fluid that rises to the top of the tank may cool as it circulates through the tubes and returns at the bottom of the tank. These tubes, fins or radiators provide additional cooling surfaces beyond those provided by the tank walls alone. Fans may also be provided to force a current of air to blow across the heated transformer enclosure, or across radiators or tubes to better transfer the heat from the hot fluid and heated tank to the surrounding air. Also, some transformers include a forced oil cooling system which includes a pump to circulate the dielectric coolant from the bottom of the tank through pipes or radiators to the top of the tank (or from the tank to a separate and remote cooling device and then back to the transformer).
To effectively transfer heat away form the transformer core and coil assembly so as to maintain an acceptably low operating temperature, conventional transformers require relatively large volumes of dielectric fluid. For example, a standard 15 kVA pole mounted single phase distribution transformer housed in a cylindrical container and having a head space of air above the fluid may contain approximately ten gallons of fluid. Every gallon of fluid increases the weight of the transformer by approximately eight pounds. Thus, for the example given above, the fluid alone adds over eighty pounds to the transformer. The weight of the dielectric fluid also may require that a transformer enclosure be made of heavier gage steel than would be required for a smaller transformer, or may require that special or stronger hangers or supports be provided. Such additions also increase the weight and cost of the transformer. Obviously then, there are cost advantages and weight savings that can be obtained from a transformer design that will effectively dissipate heat using less-than-conventional volumes of dielectric coolant.
Obviously, the more dielectric fluid that must be utilized to effectively dissipate the heat in a transformer, the larger the transformer tank or enclosure must be. Unfortunately, increasing the size of the transformer has undesirable consequences even beyond the size and weight considerations discussed above. First, transformers, particularly the common pole mounted distribution transformers, are frequently mounted in areas congested by other electrical distribution equipment, including other transformers, conductors, fuses, and surge arrester, as well as by telephone and cable TV lines and cables. Important minimum clearances must be maintained between the energized transformer terminals and all other nearby equipment and lines and all grounded structures, including the transformer""s own grounded tank. Accordingly, because of the height of conventional transformers, a dimension that, in great part, is dictated by the fluid volume required in the application, maintaining the appropriate clearance is ever-increasingly becoming a problem when trying to locate and mount the transformer.
Other significant drawbacks are directly associated with the size and weight of conventional transformers. Providing a transformer design that is smaller and lighter than conventional, similarly-rated transformers would save costs associated with shipping and storing larger and heavier equipment, and may ease installation difficulties and lessen installation costs given that a smaller transformer may not require the same equipment or personnel to install as a larger, heavier unit.
In many instances, however, reductions in the size of a transformer are limited by the effectiveness of the dielectric coolant. Many properties of a dielectric coolant affect its ability to function effectively and reliably. These include: flash and fire point, heat capacity, viscosity over a range of temperatures, impulse breakdown strength, gassing tendency, and pour point.
The flash and fire point of the fluid, as determined by ASTM D-92, are critical properties of a dielectric fluid. The flash point represents the temperature of the fluid that will result in an ignition of a fluid""s vapors when exposed to air and an ignition source. The fire point represents that temperature of the fluid at which sustained combustion occurs when exposed to air and an ignition source. It is preferred that the flash point of a transformer fluid intended for general use be at least about 145xc2x0 C. for reasonable safety against the various hazards inherent with low flammable fluids. Fluids intended for high fire point applications should have a fire point of at least about 300xc2x0 C. in order to meet current specifications for high fire point transformer fluids.
Because dielectric fluids cool the transformer by convection, the viscosity of a dielectric coolant at various temperatures is another important factor in determining its effectiveness. Viscosity is a measure of the resistance of a fluid to flow. The flowability of dielectric coolants is typically discussed in terms of its kinematic viscosity, which is measured in stokes and is often referred to merely as xe2x80x9cviscosity.xe2x80x9d The kinematic viscosity measured in stokes is equal to the viscosity in poises divided by the density of the fluid in grams per cubic centimeter, both measured at the same temperature. In the balance of this discussion, xe2x80x9cviscosityxe2x80x9d will refer to kinematic viscosity. With other factors being constant, at lower viscosities, a transformer fluid provides better internal fluid circulation and better heat removal. Organic molecules having low carbon numbers tend to be less viscous, but reducing the overall carbon number of an oil to reduce its viscosity also tends to significantly reduce its fire point. The desired insulating fluid possesses both an acceptably low viscosity at all temperatures within a useful range and an acceptably high fire point. A preferred dielectric coolant will have a viscosity at 100xc2x0 C. no higher than 15 cS, and more preferably below 12 cS.
The pour point of a fluid also affects its overall usefulness as a dielectric coolant, particularly with regard to energizing equipment in cold climates. A pour point of xe2x88x9240xc2x0 C. is considered to be an upper limit, while a maximum of about xe2x88x9250xc2x0 C. is preferred. Pour point depressants are known, but their use in transformer fluids is not preferred because of the possibility that these materials may decompose in service with time. Also, even with the use of a pour point depressant, it may not be possible to achieve the desired pour point. Therefore, it is preferred that the unmodified transformer fluid have an acceptable pour point.
The gassing tendency of a dielectric coolant is another important factor in its effectiveness. Gassing tendency is determined by applying a 10,000 volt a.c. current to two closely spaced electrodes, with one of the electrodes being immersed in the transformer fluid under a controlled hydrogen atmosphere. The amount of pressure elevation in the controlled atmosphere is an index of the amount of decomposition resulting from the electrical stress that is applied to the liquid. A pressure decrease is indicative of a liquid that is stable under corona forces and is a net absorber of hydrogen.
Other important properties of dielectric coolants are as follows. A fluid""s dielectric breakdown at 60 Hz indicates its ability to resist electrical breakdown at power frequency and is measured as the minimum voltage required to cause arcing between two electrodes submerged in the fluid. A fluid""s impulse dielectric breakdown voltage indicates its ability to resist electrical breakdown under transient voltage stresses such as lightning and power surges. The dissipation factor of a fluid is a measure of the dielectric losses in that fluid. A low dissipation factor indicates low dielectric losses and a low concentration of soluble, polar contaminants.
In the past, various polychlorinated biphenyl (PCB) compositions have been used as dielectric coolants in transformers and other apparatus in order to overcome fire safety problems. PCB""s have fallen into disfavor, however, due to their toxicity and capacity for environmental damage, detriments which are compounded by their resistance to degradation. Therefore, a suitable alternative to PCB""s is desired. A suitable dielectric coolant must possess not only acceptable electrical and physical properties, but must also be less flammable as evidenced by a high fire point, be environmentally compatible, and be reasonably priced. Various substitutes for the PCB""s have been proposed, but all are deficient as to one or more of these requirements.
Dimethyl silicone meets certain of the requirements for transformer fluids, but it is considered very expensive and is nonbiodegradable. It is also known to use hydrocarbon oils as dielectric coolants, but they are significantly deficient in some properties. For example, high molecular weight hydrocarbon oils that have fire points over 300xc2x0 C. tend to have high pour points, in the range of 0xc2x0 to xe2x88x9210xc2x0 C., and therefore cannot be used in electrical equipment that is exposed to low ambient temperatures. On the other hand, low molecular weight mineral oils have lower pour points, but have fire points of well below 300xc2x0 C. Some paraffinic oils have high fire points but also have unacceptably high viscosities and pour points. Likewise, while some naphthenic oils are suitably non-viscous, they tend to have low fire points and high pour points.
Because of these varying properties, mineral oils used as dielectric fluids are typically defined by their refined properties rather than by a defined composition. Naturally-occurring mineral oils vary in their composition based upon crude oil source and refining process. Additives are often required to make this refined product acceptable. More importantly, and especially so in recent years, the safety and environmental acceptability of mineral oils has come into question. Because mineral oils contain thousands of chemical compounds, it is impossible from a chemical and toxicological perspective to define accurately the composition and environmental effects of mineral-based oils. Therefore, it is desirable to provide a transformer fluid that comprises only a few, known chemicals, each of which is proven to be environmentally safe.
In addition, moisture, oxygen and environmental pollutants detrimentally affect the characteristics of dielectric fluids. Specifically, moisture reduces the dielectric strength of the fluid, while oxygen helps form sludge. Sludge is formed primarily due to the decomposition of mineral oil resulting from the oil""s exposure to oxygen in the air when the fluid is heated.
To prevent such contaminants from entering the transformer tank, it is common practice to include a gasketed lid or cover on the transformer. A removable cover permits the transformer to be serviced, while the rubber gasket is intended to protect the integrity of the dielectric fluid; however, such gaskets are not the surest protection from contamination by moisture, oxygen or pollutants. For example, such gaskets are known to dry and crack with age. Further, some such cover assemblies are designed to function as a pressure relief means so as to relieve excessive pressure that may form within the transformer tank as the temperature rises. Sometimes a gasket will not properly reseal itself after a release. Likewise, the gasket may be misaligned or improperly installed when, for example, the cover is removed and replaced by service personnel.
As described briefly above, due to changes of temperature within the transformer enclosure, the volume of the headspace and of the fluid in the transformer tank will change. This produces a xe2x80x9cbreathingxe2x80x9d or interchange of gas through the gasketed cover, as described above, or through another type of vent or pressure relief mechanism that typically is formed in the top of the transformer tank or cover. While a rise in temperature may cause the transformer to vent gas from the headspace outside the transformer, the lowering of temperature may draw air, oxygen and moisture into the tank. The breathing may also result in the lowering of the temperature of the enclosed air to a dew point, resulting in condensation of water vapor within the tank. The gradual accumulation of quantities of moisture will decrease the insulating quality of the dielectric fluid. Also, large drops of water may collect and, being heavier than oil, will fall towards the bottom of the transformer. These large drops of water may themselves displace dielectric fluid at such a location as to cause a breakdown in insulation and a resulting short circuit. Further, on occasion, an excessive temperature rise may cause a measure of dielectric fluid to be expelled from the transformer tank through the pressure relief device. This event may produce not only undesirable environmental consequences, but it also will decrease the transformer""s capacity to dissipate heat. Depending upon such factors as the transformer""s nominal fluid capacity, the volume of fluid lost during the overpressure event, the cumulative fluid losses from other such events, and the loading on the transformer, the life of the transformer may be significantly shortened by an increase in operating temperature caused by the loss of dielectric fluid.
Accordingly, despite the advances made in transformer and dielectric fluid technology, there remains a need in the art for a transformer that is smaller, lighter weight and that contains less dielectric coolant than conventional transformers. Preferably, the transformer enclosure would be completely and permanently hermetically sealed and non-venting such that no air, moisture or other environmental pollutants could enter the transformer and contaminate the dielectric fluid. Such a transformer should also prevent dielectric fluid from being expelled, thus protecting the environment and ensuring that the transformer""s ability to self-cool will not be diminished. The dielectric fluid preferably should have a defined chemical composition and have no adverse environmental consequences. It would be especially desirable if the transformer would have a reduced height compared to conventional transformers so as to provide additional clearance. These and other objects and advantages of the invention will appear and be understood from the following description.
The invention advances the present day technology relating to transformers and other fluid-containing electrical apparatus. The invention provides an electrical apparatus having an expandable chamber that is permanently sealed from the ambient environment. The chamber contains a transformer core and coil assembly (or other current carrying conductor) in the sealed chamber and includes a dielectric liquid completely filling the chamber. The liquid is sealed in the chamber at an absolute pressure that is less than one atmosphere. It is preferred that the enclosure have flexible walls that are interconnected to form a noncylindrical enclosure having a polygonal cross-sectional area. No service port, gasketed cover or vent means is provided in the preferred enclosure. Instead, the sides of the enclosure flex inwardly and outwardly (toward the core and coil assembly and away from the core and coil assembly, respectively) as the dielectric fluid expands and contracts. Preferably, the chamber is allowed to expand to have a volume at least 10 to 15% greater than the volume possessed by the chamber when it is initially filled and sealed. Preferably, the dielectric fluid is sealed in the chamber at a pressure of about 1 to 7 p.s.i. below atmospheric pressure, and most preferably about 1 to 3 p.s.i. less than atmospheric pressure.
A duct may be provided in the internal chamber forming a fluid passageway for directing dielectric fluid that has been heated by the submerged core and coil assembly toward the top of the enclosure. The duct also provides at least one second fluid passageway for directing the descending, cooler fluid it drops toward the bottom of the enclosure. The duct provides for a smooth laminar flow of dielectric fluid within the enclosure and reduces fluid turbulence, thereby permitting the transformer to better dissipate the heat generated as a result of transformer losses. In one embodiment of the invention, the duct includes a chimney that surrounds the core and coil assembly and includes insulative standoffs forming longitudinally-aligned channels. The standoffs prevent the inwardly flexing sides of the transformer enclosure from obstructing the fluid passageways that convey the dielectric fluid. In an alternative embodiment, the duct comprises a plurality of strip members preferably attached in one or more corners of the polygonal enclosure. Such strips divide the chamber between a first, inner fluid passageway for conducting heated fluid toward the enclosure top and a plurality of outer fluid passageways for directing the cooler fluid as it drops toward the bottom of the tank. It is preferred that such strips be attached to the enclosure along only one of their edges to allow the enclosure sides the desired degree of flexure.
The dielectric fluid of the present invention comprises a mixture of hydrocarbons having a well-defined chemical composition. The physical properties of the blend can be tailored to meet the requirements of use in various electrical power distribution equipment, and in transformers in particular. The dielectric coolants of the present invention are particularly suited for use in sealed, non-vented transformers, and have improved performance characteristics as well as enhanced safety and environmental acceptability. The present dielectric coolants comprise relatively pure blends of compounds selected from the group consisting of aromatic hydrocarbons, polyalphaolefins, polyol esters, and natural vegetable oils.
The invention further includes a method for constructing a transformer that is completely filled with a dry, degassed dielectric fluid having a desired chemical composition. According to the invention, the fluid is filtered, dried and degassed. A vacuum is drawn in the transformer enclosure and, while maintaining a sub-atmospheric pressure in the transformer enclosure, the transformer is filled with the dried and degassed fluid. The transformer is then permanently sealed. Preferably, the fluid is dried to less than 10 ppm H2O and degassed to less than 100 microns of Hg prior to the transformer being filled.
To ensure that no gas enters the transformer enclosure while it is being filled, the preferred filling method includes the steps of providing a first wet header and a second wet header that has a larger volume than the first wet header, filling the first wet header and a portion of the second wet header with a predetermined volume of dried and degassed fluid while leaving a headspace in the second wet header, drawing a partial vacuum in the headspace of the second wet header, circulating the predetermined volume of fluid between the first and second headers, and transferring a measure of the predetermined volume of fluid from the first wet header into the transformer. Ensuring that substantially all gas is removed from the fluid before the transformer is filled greatly enhances the ability of the fluid and the transformer to dissipate heat and to do so with substantially less dielectric fluid than employed in a conventional transformer.
Thus, the present invention comprises a combination of features and advantages which enable it to substantially advance the art of transformer design and manufacture and related technologies by providing a completely and permanently hermetically sealed transformer and a preferred dielectric fluid that can not become contaminated or degrade due to the entrance of moisture, air or other pollutants. The transformer is substantially smaller and much lighter in weight than conventional transformers of equal rating. The device is significantly shorter than similarly-rated conventional transformers and thus may be installed in locations where maintaining the appropriate clearance from wires and other apparatus would otherwise be impossible or exceedingly difficult. The invention requires substantially less dielectric fluid than a conventional transformer, yet is able to adequately dissipate heat so as to avoid excessive temperature rise and premature transformer failure. The transformer prevents any dielectric fluid from being expelled and further employs a fluid having a defined chemical composition and having no adverse environmental consequences.
These and various other characteristics and advantages of the present invention will be readily apparent to those skilled in the art upon reading the following detailed description and referring to the accompanying drawings.